The overall objective of the proposed research is to define the association between subclinical hypothyroidism and prolactin activity and human breast cancer. Attention will be paid to the role which these factors may have both in the etiology of breast cancer and the prognosis. The objective will be accomplished by determining plasma TSH, T3, T4 and prolactin levels, and testing for thyroid antibodies, in two clinical situations: (1) in apparently healthy women who are taking part in a breast cancer screening program, and (2) in patients awaiting mastectomy or about to be entered in a clinical trial of adjuvant therapy. The endocrine studies will be complemented by appropriate follow-up procedures. The results will be correlated with ongoing studies of steroid hormones in these patients.